The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal cord and nerves. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebral body including a portion of relatively weak cancellous bone and a portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces experienced by the spinal column. A vertebral canal containing the spinal cord and nerves is located posterior to the vertebral bodies. In spite of the complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. For example, the kinematics of the spine normally includes flexion, extension, rotation and lateral bending.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in the lumbar or cervical spine), and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain, as well as diminished nerve function. These spinal disorders, pathologies, and injuries limit the spine's range of motion, or threaten the critical elements of the nervous system housed within the spinal column.
The treatment of acute and chronic spinal instabilities or deformities of the thoracic, lumbar, and sacral spine has traditionally involved rigid stabilization. For example, arthrodesis, or spine fusion, is one of the most common surgical interventions today. The purpose of fusion or rigid stabilization is the immobilization of a portion of the spine to affect treatment. Rigid stabilization typically includes implantation of a rigid assembly having metallic rods, plates and the like that secure selective vertebrae relative to each other. Spinal treatment using rigid stabilization, however, does have some disadvantages. For example, it has been shown that spine fusion decreases function by limiting the range of motion for patients in flexion, extension, rotation and lateral bending. Furthermore, it has been shown that spine fusion creates increased stresses and therefore, accelerated degeneration of adjacent non-fused segments. Another disadvantage of fusion is that it is an irreversible procedure.
More recently, dynamic stabilization has been used in spinal treatment procedures. Dynamic stabilization does not result in complete spinal fusion but instead permits enhanced mobility of the spine while also providing sufficient stabilization to effect treatment. One example of a dynamic stabilization system is the Dynesys® system available from Zimmer Spine, Inc. of Edina, Minn. Such dynamic stabilization systems typically include a flexible spacer positioned between pedicle screws installed in adjacent vertebrae of the spine. Once the spacer is positioned between the pedicle screws, a flexible cord is threaded through eyelets formed in the pedicle screws and a channel through the spacer. The flexible cord retains the spacer between the pedicle screws while cooperating with the spacer to permit mobility of the spine.
Many current dynamic stabilization systems are typically assembled in situ. In these systems, a surgical site is established in the patient and a pair of bone anchors is coupled to adjacent vertebrae. Spacers are then inserted into the surgical site and positioned between the anchors while a flexible cord is threaded through the anchors and spacers, generally in a direction parallel to the axis of the spine, to assemble the device in the body. Once the stabilization system is assembled, the appropriate amount of pre-tensioning must be applied to the cord and other post-assembly adjustments must be made to effect spinal treatment.
While dynamic stabilization systems are generally successful for treating various spinal conditions, manufacturers or providers of such stabilization systems continually strive to improve these stabilization systems. By way of example, manufacturers or providers strive to provide relatively quick and convenient assembly and installation of the stabilization system. For example, minimally invasive surgical techniques often utilize much smaller incisions and provide the benefits of less tissue and muscle displacement and quicker recovery.
Manufacturers or providers of stabilization systems also strive to provide systems that transmit imposed loads on the spine through the system and to the underlying bone structure in an efficient and effective manner. In such applications, for example, engagement of the spacers with the connectors on the anchors should provide for optimal transmission of the loads imposed on the stabilization systems to the underlying bone structure. Ideally, when the stabilization system is assembled, the end faces of the spacer will substantially mate with the surfaces of the eyelet so as to maximize the contact area between the spacer and the anchor.
If the contact between the ends of the spacer and the surfaces of the eyelets occurs at less than the full contact area results, then such a reduction in contact area between the components localizes the load transfer. This may be due to the specific vertebral physiology to which the stabilization system is being applied, the geometry of the components or the non-idealized placement of the anchors in the vertebrae. In any event, the resulting reduction in contact area between the spacer and anchors may diminish the capacity of the stabilization system to efficiently transmit applied loads to the vertebrae to which the anchors are attached. This may result in a reduction in the support provided by the stabilization system, a loss of the pre-tensioning of the system, or otherwise affect the stabilization system in a manner that impacts treatment of the spine.
Accordingly, there is a need for an improved dynamic stabilization system and method of using the same that addresses these objectives.